1. Technical Field
This invention relates to electrolytic cells. In one aspect, this invention relates to cathode collector bars of electrolytic reduction smelting cells used in the production of aluminum.
2. Background
Aluminum is produced by an electrolytic reduction of alumina in an electrolyte. The aluminum produced commercially by the electrolytic reduction of alumina is referred to as primary aluminum.
Electrolysis involves an electrochemical oxidation-reduction associated with the decomposition of a compound. An electrical current passes between two electrodes and through molten Na3AlF6 cryolite bath containing dissolved alumina. Cryolite electrolyte is composed of a molten Na3AlF6 cryolite bath containing alumina and other materials, e.g., such as fluorspar, dissolved in the electrolyte. A metallic constituent of the compound is reduced together with a correspondent oxidation reaction.
Electrical current is passed between the electrodes from an anode to a cathode to provide electrons at a requisite electromotive force to reduce the metallic constituent which usually is the desired electrolytic product, such as in the electrolytic smelting of aluminum. The electrical energy expended to produce the desired reaction depends on the nature of the compound and the composition of the electrolyte.
Hall-Heroult aluminum reduction cells are operated at low voltages (e.g. 4-5 volts) and high electrical currents (e.g. 70,000-325,000 amps). The high electrical current enters the reduction cell through the anode structure and then passes through the cryolite bath, through a molten aluminum metal pad, and then enters a carbon cathode block. The electrical current is carried out of the cell by cathode collector bars.
As the electrolyte is traversed by electric current, alumina is reduced electrolytically to aluminum at the cathode, and carbon is oxidized largely to carbon dioxide at the anode. The aluminum, thus produced, accumulates at the molten aluminum pad and is tapped off periodically. Commercial aluminum reduction cells are operated by maintaining a minimum depth of liquid aluminum in the cell, the surface of which serves as the actual cathode. The minimum aluminum depth is about 2 inches and may be 20 inches.
The alumina-cryolite bath is maintained on top of the molten aluminum metal pad at a set depth. The current passes through the cryolite bath at a voltage loss directly proportional to the length of the current path, i.e., the interpolar distance gap between the anode and molten aluminum pad. A typical voltage loss is about 1 volt per inch. Any increase of the anode to cathode spacing restricts the maximum power efficiency and limits the efficiency of the electrolytic cell operation.
Much of the voltage drop through an electrolytic cell occurs in the electrolyte and is attributable to electrical resistance of the electrolyte, or electrolytic bath, across the anode-cathode distance. The bath electrical resistance or voltage drop in conventional Hall-Heroult cells for the electrolytic reduction of alumina dissolved in a molten cryolite bath includes a decomposition potential, i.e., energy used in producing aluminum, and an additional voltage attributable to heat energy generated in the inter-electrode spacing by the bath resistance. This latter heat energy makes up 35 to 45 percent of the total voltage drop across the cell, and in comparative measure, as much as twice the voltage drop attributable to decomposition potential.
An adverse result from reducing anode-cathode distance is a significant reduction in current efficiency of the cell when the metal produced by electrolysis at the cathode is oxidized by contact with the anode product. For example, in the electrolysis of alumina dissolved in cryolite, aluminum metal produced at the cathode can be oxidized readily back to alumina or aluminum salt by a close proximity to the anodically produced carbon oxide. A reduction in the anode-cathode separation distance provides more contact between anode product and cathode product and significantly accelerates the reoxidation or xe2x80x9cback reactionxe2x80x9d of reduced metal, thereby decreasing current efficiency.
The high amperage electrical current passing through the electrolytic cell produces powerful magnetic fields that induce circulation in the molten aluminum pad leading to problems such as reduced electrical efficiency and xe2x80x9cback reactionxe2x80x9d of the molten aluminum with the electrolyte. The magnetic fields also vary the depths in unequal distance between the molten aluminum pad and the anode. The motion of the metal pad increases, sometimes violently stirring the molten pad and generating vortices, and causing localized electrical shorting.
Metal pad depth variations restrict the reduction of the anode to cathode gap and produce a loss in current efficiency. Power is lost to the electrolyte interposed between the anode and cathode blocks. Movement of the molten aluminum metal pad also contributes to uneven wear on the carbon cathode blocks and can accelerate cell failure.
Metal pad turbulence also increases the xe2x80x9cback reaction,xe2x80x9d or reoxidation, of cathodic products, thereby lowering cell efficiency. Metal pad turbulence accelerates distortion and degradation of the cathode bottom liner through attrition and penetration of the cryolite.
In the conventional cathode today, steel cathode collector bars extend from the external bus bars through each side of the electrolytic cell into the carbon cathode blocks. The steel cathode collector bars are attached to the cathode blocks with cast iron, carbon glue, or rammed carbonaceous paste to facilitate electrical contact between the carbon cathode blocks and the steel cathode collector bars.
The flow of electrical current through the aluminum pad and the carbon cathode follows the path of least resistance. The electrical resistance in a conventional cathode collector bar is proportional to the length of the current path from the point the electric current enters the cathode collector bar to the nearest external bus. The lower resistance of the current path starting at points on the cathode collector bar closer to the external bus causes the flow of current through the molten aluminum pad and carbon cathode blocks to be skewed in that direction. The horizontal components of the flow of electric current interact with the vertical component of the magnetic field, adversely affecting efficient cell operation.
Existing Hall-Heroult cell cathode collector bar technology is limited to rolled or cast mild steel sections. The high temperature and aggressive chemical nature of the electrolyte combine to create a harsh operating environment. The high melting point and low cost of steel offset its relatively poor electrical conductivity. In comparison, potential metallic alternatives such as copper or silver have high electrical conductivity but low melting points and high cost. Copper is used in the apparatus and process of the present invention because it provides a preferred combination of electrical conductivity, melting point, and cost. Other high conductivity materials could be used based on their combinations of electrical conductivity, melting point, and cost relative to the aluminum smelting process.
The electrical conductivity of steel is so poor relative to the aluminum metal pad that the outer third of the collector bar, nearest the side of the pot, carries the majority of the load, thereby creating a very uneven cathode current distribution within each cathode block. Because of the chemical properties, physical properties, and, in particular, the electrical properties of conventional anthracite cathode blocks, the poor electrical conductivity of steel had not presented a severe process limitation until recently.
Conventional cathodes contained either 100% Gas Calcined Anthracite (GCA) or 100% Electrically Calcined Anthracite (ECA). These cathode blocks had poor thermal shock resistance. These cathode blocks swelled badly under electrolysis conditions, i.e., under the influence of cathodic current, reduced sodium, and dissolved aluminum. These cathode blocks had poor electrical conductivity (relative to graphite). In their favor, these cathode blocks had low erosion or wear rates (relative to graphite).
To overcome the shortcomings of 100% anthracite cathodes, cathode manufacturers added an increasing proportion of graphite to the raw cathode block mix. A minimum of 30% graphite seems to be sufficient to avoid thermal shock cracking and to provide reasonable electrical properties and sodium resistance in most instances. Further additions up to 100% graphite aggregate or 100% coke aggregate graphitized at 2,000-3,000xc2x0 C. provide preferred operating and productivity conditions.
As the graphite content or degree of graphitization increases, the rate increases at which the cathode blocks erode or are worn away.
In pursuit of economies of scale, aluminum smelting pots have increased in size as the operating amperage has increased. As the operating amperage has been increased, the percentage of graphite in cathodes has increased to take advantage of improved electrical properties and maximize production rates. In many cases, this has resulted in a move to graphitized cathode blocks.
The operation of the pot is most typically terminated when the aluminum metal is contaminated by contact with the steel collector bars. This can happen when the cathode to seam mix joints leak, when the cathode blocks crack or break because of thermal or chemical effects or the combined thermochemical effects, or when erosion of the top surface of the block exposes the collector bar. In the application of higher graphite and graphitized cathode blocks, the dominant failure mode is due to highly localized erosion of the cathode surface, eventually exposing the collector bar to the aluminum metal.
In a number of pot designs, higher peak erosion rates have been observed for these higher graphite content blocks than for 30% graphite/ECA blocks or 100% ECA blocks. Operating performance is therefore traded for operating life.
There is a link between the rapid wear rate, the location of the area of maximum wear, and the non-uniformity of the cathode current distribution. The higher graphite content and graphitized cathodes are more electrically conductive and as a result have a much more non-uniform cathode current distribution pattern and hence higher wear rate.
Accordingly, there is a need to develop and provide a more even cathode current distribution so that the cathode wear rate will be decreased, the pot life will be increased, and the operating benefits of the higher graphite and graphitized cathode blocks can be realized.
A related objective of the present invention is to provide an electrolytic reduction cell apparatus and method utilizing a novel cathode collector bar, including a solid, ferrous metal spacer for maintaining a controlled heat balance in the pot.
These and other objects of the present invention will become more apparent from reference to the following detailed description of our invention.
In accordance with the present invention, there is provided an apparatus and method for production of aluminum. The apparatus of the invention comprises an electrolytic cell for reducing alumina dissolved in a molten salt bath to aluminum metal. An electric current passes between an anode and a cathode through the molten bath, producing aluminum metal adjacent the cathode.
The electrolytic cell has cell walls including a first cell wall and a second cell wall, an anode, a carbonaceous cathode block separated from the anode, a bus bar external to the first cell wall, and a collector bar connecting the bus bar with the cathode block. The cathode block preferably defines a slot in which the collector bar is seated. The cell walls define a chamber containing a molten salt bath.
The collector bar includes a ferrous metal body and a copper insert. As used herein, the term xe2x80x9cferrous metalxe2x80x9d refers to iron and steel, including mild steel, low carbon steel, and stainless steel. The term xe2x80x9ccopperxe2x80x9d includes alloys of copper with various other metals including silver. For practice of the present invention we prefer relatively pure forms of copper containing at least 99 wt. % copper because of their excellent electrical conductivity.
The collector bar has a ferrous metal body comprising a solid, ferrous metal spacer having an external end portion connected with the bus bar and an internal end portion spaced inwardly of the first cell wall. The spacer improves heat balance in the cell by preventing excessive heat transfer between the copper insert and the bus bar. The ferrous metal body also includes a ferrous metal sheath integral with the spacer and defining a cavity containing the copper insert. The cavity extends between an external end adjacent the internal end portion of the spacer, and an internal opening. The cavity and the copper insert may be polygonal or circular in transverse cross-section. We prefer a cylindrical copper insert inside a cavity having a circular transverse cross-section.
The slot in the cathode block preferably contains means for joining the collector bar to the cathode block, preferably an electrically conductive material. This material may be cast iron, carbonaceous glue or rammed carbonaceous paste and is preferably cast iron.
The cathode assembly of the present invention is useful for producing aluminum by electrolysis. The cathode assembly is spaced downwardly of an anode in a chamber containing a molten salt bath. An electric current passes from the anode to the cathode assembly, reducing alumina dissolved in the molten salt bath to aluminum deposited in a pad above the cathode block. The copper insert in the collector bar distributes electric current more evenly than in prior art cells having collector bars containing only steel or other ferrous metal. The ferrous metal spacer in the collector bar body reduces heat losses, compared with collector bars having a copper insert connected directly to the bus bar.